Various reconstructive surgical procedures, such as anterior cruciate ligament (ACL) reconstruction in the knee and rotator cuff repair, require healing between a tendon and bone. Previous animal studies found that normal bone-tendon attachment-site morphology is not formed during healing, resulting in a mechanically inferior interface. Little is known of the cellular and molecular events that initiate and regulate the healing process, and how mechanical stimuli affect these events. Ideally, healing between tendon and bone should recapitulate the process that occurs during embryonic development. The long-term objective of this research is to understand the cellular and molecular events that control healing at the tendon-to-bone attachment site, so that we may eventually develop novel methods for improving the healing process. We will test hypotheses about tendon-to-bone healing drawn from current knowledge of insertion site embryogenesis and the role of inflammation in tissue healing. Although formation and maintenance of joint structures is dependent on muscular activity, the initial positioning and cellular changes during embryonic joint development appear to be specified prior to and therefore independent of muscle activity. Thus, our global hypothesis is that optimal attachment-site healing is controlled by both genetic (intrinsic) and mechanical (extrinsic) factors. Specifically, we hypothesize that mechanical load causing relative tendon- bone interface motion (shear) will improve healing only if applied after an initial period of immobilization to allow resolution of acute inflammation and initiation of early healing (i.e., early vs delayed mobilization). To test this hypothesis, we will: (1) Evaluate the effect of the timing and magnitude of motion on tendon-to- bone healing, (2) Evaluate the mechanism(s) by which mechanical stimulus affects healing, and (3) Determine the role of macrophages in tendon-to-bone healing by evaluating the effect of macrophage depletion on healing. Understanding the intrinsic and extrinsic factors that influence tendon-to-bone healing could have a tremendous impact on improving the outcomes of ACL reconstruction and rotator cuff repair, two of the most commonly performed orthopaedic procedures today. Altered intraoperative techniques or postoperative rehabilitation protocols that are based on a better understanding of tendon to bone healing could directly and significantly improve long-term clinical outcomes. [unreadable] [unreadable] [unreadable]